The point of departure for the invention is an automatic alignment system for the tuner of a television receiver with a controlled high-frequency preamplifier stage and an intermediate-frequency amplifier that has a reference circuit with a reasonance frequency that can be switched to the arithmetic mean between the video-carrier intermediate frequency and the audio-carrier intermediate frequency during automatic alignment, whereby the maximum control voltage is employed as a criterion for optimum alignment and an auxiliary oscillator is switched on during automatic alignment and generates through a mixing stage an auxiliary frequency that corresponds to the arithmetic mean of the video-carrier frequency and the audio-carrier frequency from the tuner-oscillator frequency, and the automatic alignment occurs in that tuning-voltage values are sequentially applied in digital steps subject to microprocessor control through digital-to-analog converters to the capacity diodes in the high-frequency tuning circuits and the increase in the tuning-voltage values is discontinued when a maximum control voltage is attained, with the detected tuning-voltage value being retained in the activated digital-to-analog converter.
A tuner is described in German OS No. 2 854 852 for example. It uses three auxiliary oscillators and their associated analog memories in conjunction with the PLL circuit to tune the high-frequency circuit to the desired reception frequency. This tuner also exploits the tuner oscillator for alignment. Furthermore, the number of auxilliary oscillators must equal that of the high-frequency circuits to be aligned. The number of auxiliary oscillators needed and hence the materials cost will then increase especially when for example a television receiver is being aligned and bands I, III, IV, and V are to be tuned, entailing the risk of ambiguities between the frequencies of the tuner oscillator and auxiliary oscillator and the reception frequencies. One drawback to the realization of these tuners is that the exciter windings in the auxilliary oscillators can in conjunction with the individual coils of the high-frequency circuits provoke undesirable resonances and attenuations in the circuitry that in turn occasion uncontrollable mistuning in the circuits when the auxiliary oscillators are switched off once the alignment is finished. The tuner also involves analog memories that must be recharged at intervals in order to maintain the optimum tuning values. Optimum alignment of the receiver is accordingly impossible in principle. Furthermore, the constantly repeated retuning results in disturbances that can be seen on the screen and heard in the loudspeaker.
The journal Nachrichten Elektronik 11 (1979), 365-68 describes a method of tuning radio receivers that also employs additional auxilliary circuits in the high-frequency filters. This method also results in deleterious effects on the circuits once the aligned receiver is in operation. As the left-hand and middle columns on page 367 say, precise tuning can only be attained by means of special components in the form of exactly paired diodes. This also entails a not inconsiderable expense.
A system of testing receivers is known from U.S. Pat. No. 3,736,512 that in on-line operation monitors the properties of the various stages of a receiver--the preamplifier, mixing stage, and intermediate-frequency amplifier for example. When deviations from reference values that are visually displayed are detected, they must be corrected by the operator of the receiver. This is not a closed system wherein high-frequency filter circuits are automatically optimally aligned for each reception channel.